1. Field of the Invention
The invention pertains to a method and a device for locating a metal line through the coupling of a transmitted signal into the line, and the receiving and evaluation of the level of a received signal radiated by the line.
2. Discussion of Related Art
Methods and devices are known that can be used to locate the course of a metal and electrically conductive line (cable, conduit) when an electric current flows through the line. In those devices, the electromagnetic field from this current is detected, processed and displayed. In the case of a method for improving the locating of a line, a transmitter is used whose defined signal is coupled into the line capacitively, inductively or conductively and causes a current flow in the line. As a result, the locating of the line can be carried out with a receiver which is optimized for detecting the defined signal from the transmitter.
Locating a line with relatively simple methods and devices is possible if the line is run individually. In practice, however, the problem often exists that several lines run alongside each other with comparatively little space between them. Even though the signal from the transmitter is connected to only one line, in the course of the line this primary signal can be coupled capacitively, inductively or conductively into one or more adjoining lines. The secondary current flowing in these other lines then leads to faulty locating.
An improvement of line locating can be achieved if the receiver additionally has at its disposal means for measuring and displaying the run depth of the line and the resulting current in the line. However, falsification of these measured values occurs whenever the fields from several lines superimpose on each other and strengthen or weaken the resulting field depending on the direction of the signal flow. It is thus impossible to distinguish two lines from each other if the primary transmission current is flowing in the one line to be located and the circuit is connected to the transmitter via a second line, caused, for example, by common circuits for the potential equalization.
A further improvement in line locating can be achieved if the signal from the transmitter contains an identifier from which the signal flow direction from or to the transmitter can be unequivocally determined.
In U.S. Pat. No. 5,260,659, a method and device are described using a transmitter which generates two frequencies, whereby a first frequency is an integral multiple of a second frequency. In an additional development of the device, a first frequency and a second frequency are generated, to which a subharmonic frequency of the first frequency is added or subtracted. To locate the line, the receiver evaluates the phase relationships of the frequencies, thus making it possible to determine the direction of the signal flow.
A similar instrument for locating a line in a network of multiple lines is described in U.S. Pat. No. 5,438,266. Using a transmitter, an even-numbered harmonic and an odd-numbered harmonic of a frequency are generated and connected to the line to be located. A receiver includes antennas and filters that are tuned to these frequencies. The locating of the line can be carried out through evaluation of the phase relationships, particularly the reversal of the phase relationships.
A substantial disadvantage of the methods and devices mentioned above is that reliable locating is successful only with comparatively short lengths of the line to be located. This disadvantage is based on the various changes of the phase relationships of the frequencies which are brought about by parasitic complex components of the line, for example, the self-capacitance or the self-inductance of the line. Moreover, there is a relatively high expense involved in the realization of a sufficiently great selectivity of the receiver""s antennas and filters, each of which has to be separately optimized for one of the frequencies generated by the transmitter.
Another device for locating the course of lines is disclosed in DE 3708968. There, an identifier running in the direction of the current is superimposed on the signal from a transmitter. A special receiver evaluates the identifier and thus makes it possible to determine the direction of the current. The identification is carried out by superimposing the transmitted signal with unipolar pulses that are synchronized with the transmitted signal. In a further development of the device, the transmitted signal is broken into time-limited segments made up of at least one sine period, whereby the start of a segment after a transmission pause takes place with a defined edge of the sinusoidal signal. A substantial disadvantage of this device lies in the fact that the receiver for receiving the asymmetric and/or non-periodic signal has to be dimensioned to be relatively broadband, and so, as a result of the low selectivity, a substantial effect is possible from interference frequencies such as upper harmonics from the power supply network.
All of the previously mentioned methods and devices have the disadvantage that the use of several different periodic signals or asymmetric signals make for greater expense in the receiver in order to realize adequate selectivity with regard to interference influences, and that as a result of the influence of the complex parameters of the line such as the capacitance or the inductance, a dependency exists between the length of the line and the phase shift of the signal.
According to the present invention, a method and a device for locating the course of a metal line is presented. With a method and device according to the present invention, a reliable identification of a line and an exact locating of the course of the line is possible, particularly in the case of multiple lines running in parallel.
In one embodiment, the transmitted signal is generated through the modulation of a first carrier frequency with a second, lower frequency. The received signal detected at the receiver is evaluated with regard to its period and its polarity, which is dependent on the orientation of a receiving antenna relative to the line. The evaluation results in a signaling of xe2x80x9coutgoing transmitted signalxe2x80x9d or xe2x80x9cincoming transmitted signalxe2x80x9d with regard to the direction of the signal flow in the line.
A transmitter according to the present invention includes a first generator having a first carrier frequency, a second generator having a second, lower frequency, and a modulator that modulates the first frequency with the second frequency. The corresponding receiver includes an evaluation device that evaluates the received signal with regard to its period and its polarity, which is dependent on the orientation of a receiving antenna relative to the line. The evaluation device addresses a second signaling unit for an xe2x80x9coutgoing signalxe2x80x9d and a third signaling unit for an xe2x80x9cincoming signalxe2x80x9d.
The advantages of most embodiments of devices and methods in accordance with the invention result from the easily realized high selectivity of the receiver, the independence of the special, complex electrical parameters and the length of the line, and the unequivocal evaluation of the received signals, independently of the type of coupling of the transmitted signal from the transmission-carrying primary line to other secondary lines. Additional advantages result from the very wide range of the frequency band, whereby no major restrictions exist with regard to the frequency and the bandwidth. A simple adaptation of the method and the device is possible by choosing the fundamental frequency of the transmitted signal according to the type of line to be located with its special, complex electrical parameters. Thus, advantages result if the fundamental frequency is relatively low for lines with great self-capacitance, such as insulated conduits, for example, but the fundamental frequency is relatively high in branched networks with short cables.
Further details, features and advantages of the invention result from the following description, aided by the drawing, of a preferred form of implementation of the invention. The following are shown: